The present invention relates to an improved automatic transmission control apparatus for a vehicle and control method thereof, in which it is possible to determine gradient of a road and change control characteristic to the characteristic suitable for the road gradient.
The conventional automatic transmission control apparatus for a vehicle is constructed on the assumption that a vehicle runs on a level road. Because the shift control characteristic of the control apparatus of the vehicle on an up hill road is the same as on the level road, the phenomenon called busy shift occurs. It is, therefore, very difficult to drive comfortably the vehicle. Under the condition of the busy shift, the vehicle on the road with a certain gradient is over-accelerated at a third position of a gear. While, the vehicle loses its speed due to the lack of driving force at a fourth position. Therefore, if the vehicle tries to maintain a constant speed, up-shift and down-shift are repeated frequently. Even in such a case, it is possible to run the vehicle at the 3rd position, because the up-shift is not occurred when an accelerator pedal is returned to the extent that the vehicle is not accelerated, by inhibiting the shift to the 4th position. Therefore, in a general vehicle, the busy shift is eliminated by driver's operation of the switch (an over-drive canceling switch) that inhibits the shift to the 4th position or over-drive.
However, in addition that such an operation is burdensome, some drivers do not know when the over-drive switch should be used. As a result, there are some problems such as the deterioration of driving characteristics and the deterioration of the fuel consumption due to frequent shifts.
Therefore, a control apparatus of an automatic transmission which can prevent occurrence of the busy shift has been proposed, for example, in Japanese Patent Publication No. 61-15301 (1986) or Japanese Patent Publication No. 61-48020 (1986).
Because in the proposed method the over-drive is inhibited when the road gradient by which there is the possibility of occurrence of the busy shift is detected by a road gradient detecting sensor or calculated based on an engine load, the over-drive is inhibited in a running state in which the busy shift is not actually occurred. Accordingly, the fuel consumption is deteriorated due to insignificant inhibition of the over-drive.
It is desirable to suppress the vehicle speed with putting the appropriate engine brake on by changing the gear ratio in accordance with the magnitude of the road gradient in a down hill road. In the conventional control apparatus, there is a fear that brake shoes are heated by frequent uses of a foot brake in a drive(D) range state, and finally the foot brake does not work due to the vapor lock of brake oil. To solve such a problem, an improved control apparatus has been proposed in Japanese Patent Laid-Open No.55-140617 (1980) and Japanese Patent Laid-Open No.3-9166 (1991).
However, in such a prior art, the gear range is compulsorily changed to the 2nd range or the 1st range when the road gradient required to put the engine brake on is obtained by a road gradient detecting sensor or from the load of the engine, etc. As a result, the engine speed becomes too high, and thus a noise extremely increases. Further, in the case that the engine speed becomes too low, the range in which the fuel is cut off becomes narrow and thus the fuel consumption is deteriorated.